This invention generally relates to a motor vehicle brake assembly. More particularly, the invention relates to a composite rear brake disc and drum.
Because of their enhanced operating characteristics, rear disc brake assemblies are currently used in motor vehicles as an alternative to rear drum brake assemblies. Originally, rear disc brake assemblies were only offered as an option on high performance, high priced automobiles. Now, however, rear disc brake assemblies are often specified as standard equipment on modestly priced passenger cars and light trucks.
Although rear disc brake assemblies provide superior braking characteristics over a wider range of operating temperatures, their use does have some limitations. These drawbacks are particularly apparent with respect to parking brake requirements.
When a rear disc brake assembly is used for primary regular braking service, brake engineers and designers typically use one of two methods for parking brake application. The first of these methods involves the modification of the disc brake caliper. According to this method, the disc brake caliper is adapted to include components which will compress the piston and brake pads against the brake disc when the parking brake is actuated through the parking brake lever and cable. The second method of parking brake application used with rear disc brake assemblies includes a small, conventional brake drum packaged within the hat section of the brake disc rotor. When the parking brake is applied, a small, but otherwise conventional, brake shoe is applied to engage the drum. In this second alternative, the brake drum is intended for use during parking brake applications and is not normally used as or in conjunction with the primary or disc brake assembly. This second alternative is the most popular of the parking brake methods used with rear disc brake assemblies because of its reliability, performance and low cost.
When provided with a drum in the hat section of the disc brake rotor, conventional rear brake disc and drum assemblies are one-piece, full-cast, grey iron assemblies. The known integral disc and drum rotor assemblies are rough formed in a casting mold and then have their surfaces machined to the appropriate dimensions. Machining is typically required to form the braking friction surfaces of the disc and the drum, as well as the mounting face of the rotor assembly and the wheel lug mounting holes. Obviously, this amount of machining is a laborious process and any reduction in machining time would be desirable. Another limitation of the unitarily cast disc and drum is that the casting process itself necessarily requires a rotor assembly which is heavy. As always with motor vehicles, it is desirable to reduce weight whenever and wherever possible.
With the limitations of the known rear brake disc and drum assemblies in mind, it is a primary object of the present invention to provide a rear brake disc and drum assembly which overcomes the above mentioned drawbacks.
Another object of the invention is to provide a brake disc and drum assembly which requires a minimal amount of machining and which has a reduced weight relative to known brake disc and drum assemblies. Yet another object of this invention is to enable the use of alternative materials for the grey cast iron now used for the disc plates.
A further object of the invention is to decrease the size of the drum package so as to allow for maximum sizing of the disc plates and the primary braking friction surfaces. A still further object of the invention is to provide a brake disc and drum assembly in which the drum, and its related mechanisms, can be used to stop the vehicle under dynamic conditions where there has been a partial failure in the primary brake system.
In achieving the above objectives, the present invention discloses a brake rotor which is a composite rear brake disc and drum assembly. The hat section of the assembly is formed from flat plate stock and includes portions which define the drum of the assembly. The hat section more specifically includes a mounting portion, a cylindrical skirt and a flange.
The mounting web of the hat section defines a mounting face that receives the mounting lugs of the vehicle's wheel mounting assembly. The cylindrical skirt extends from the mounting web so as to define an axis of rotation for the assembly. The cylindrical skirt also includes an inside surface that defines the cylindrical braking friction surface of the drum. This braking friction surface is concentric with the axis of rotation. Opposite of the mounting web, the cylindrical skirt terminates in the radially extending flange.
Once the hat section of the assembly has been formed, it is placed within a casting mold. In the mold, the disc plates of the assembly are cast onto the hat section so that they are supported on the flange. The disc plates are annular structures which define inboard and outboard braking friction surfaces oriented transversely of the axis of rotation.
The composite nature of the assembly has several advantages over the known prior designs. First, the assembly requires a minimal amount of machining in comparison to the prior designs. In the present invention, only the braking friction surfaces of the disc plate require machining. Another benefit of the present invention is that the assembly is lighter without compromising strength. Lighter weight is achieved by forming the hat section by a method other than casting. Two methods for forming the hat section include stamping and hydro-spinning. Both methods allow for the thickness of the hat section to be reduced of the prior cast designs. When formed by the hydro-spun method, the hat section is capable of exhibiting a section thickness which varies across its respective portions. As such, the hat section can be made with an increased thickness where the extra strength is needed, such as in the mounting web where clamping loads must be resisted, and a reduced thickness where the extra strength is not needed, such as in the flange.
Additional benefits and advantages of the present invention will become apparent to those skilled in the art to which this invention relates from the subsequent description of the preferred embodiments and the appended claims, taken in conjunction with the accompanying drawings.